This invention relates to orthopedic surgery, and more particularly to an anterior cervical fusion compression plate.
Of all animals possessing a backbone, human beings are the only creatures who remain upright for significant periods of time. From an evolutionary standpoint, this erect posture has conferred a number of strategic benefits, not the least of which is freeing the upper limbs for purposes other than locomotion. From an anthropologic standpoint, it is also evident that this unique evolutionary adaptation is a relatively recent change, and as such has not benefitted from natural selection as much as have the horizontal backbones of other animals. As a result, stresses acting upon the human backbone (or "vertebral column") are unique in many senses, and result in a variety of problems or disease states that are peculiar to the human species.
The human vertebral column is essentially a tower of bones held upright by fibrous bands called ligaments and contractile elements called muscles. There are seven bones in the neck or cervical region, twelve in the chest or thoracic region, and five in the low back or lumbar region. There are also five bones in the pelvic or sacral region which are normally fused together and form the back part of the pelvis. This column of bones is critical for protecting the delicate spinal cord and nerves, and for providing structural support for the entire body.
Between the vertebral bones themselves exist soft tissue structures--discs--composed of fibrous tissue and cartilage which are compressible and act as shock absorbers for sudden downward forces on the upright column. The discs allow the bones to move independently of each other, as well. The repetitive forces which act on these intervertebral discs during repetitive day-to-day activities of bending, lifting and twisting cause them to break down or degenerate over time.
Presumably because of humans' upright posture, their intervertebral discs have a high propensity to degenerate. Overt trauma, or covert trauma occurring in the course of repetitive activities disproportionately affect the more highly mobile areas of the spine. Disruption of a disc's internal architecture leads to bulging, herniation or protrusion of pieces of the disc and eventual disc space collapse. Resulting mechanical and even chemical irritation of surrounding neural elements (spinal cord and nerves) cause pain, attended by varying degrees of disability. In addition, loss of disc space height reduces tension on the longitudinal spine ligaments, thereby contributing to spinal instabilites such as spinal curvature, and lithesis.
The time-honored method of addressing neural irritation and instability resulting from severe disc damage have largely focused on removal of the damaged disc and fusing the adjacent vertebral elements together. Removal of the disc relieves the mechanical and chemical irritation of neural elements, while osseous union (bone knitting) solves the problem of instability.
In the cervical spine, the most common type of fusion utilizes either bone dowels (Cloward Technique) or bone blocks (Smith Robinson Technique). These procedures have been used now for over four decades. One of the main causes of failure of these fusion techniques is the failure to fuse, or non-union, at the site where the bone is grafted between the vertebral bodies. In an attempt to circumvent this problem, various plate-type mechanisms have been used both to provide immediate stability, and to reduce or eliminate movement at the site of the fusion to allow successful bone knitting, much as a cast on a fractured limb provides support until healing can occur.
It is recognized that for bone knitting to occur, the interfaces of bone required to knit or heal must be held in close apposition and motion between the knitting or fusion interfaces must be restricted sufficiently for a certain minimal time period to permit stable bone growth to occur.
To achieve these ends, prior inventors have developed a variety of both external braces and internal fixation instruments, some in the form of plates. Internal fixation is advantageous in that is obviates the need for cumbersome external braces, collars or supports and ensures essentially total compliance. U.S. Pats. No. 5,041,113, 5,234,431, 5,344,421, and 5,681,311 provide examples of prior vertebral bone plate systems. U.S. Pat. No. 3,604,414 discloses a plate for setting fractures, having separate elements which are attached to respective bone fragments and have a toothed interface to maintain the position of the fragments after they have been drawn together.
While interface apposition and retardation of motion are known to enhance bone healing, it is also recognized that if the bony surfaces to be fused are held together under a compression force, osseous union is further enhanced. While many plating systems maintain bony apposition and provide stability, few provide sufficient compression to be called compression plates, and none provide adjustable degrees of compression of the grafted bone.